1. Field of the Invention
The present invention relates generally to equipment used in semiconductor processing. More particularly, the present invention relates to an extreme ultraviolet lithography system in which heat is transferred from a mirror to a heat exchanger through a liquid metal interface.
2. Description of the Related Art
Extreme ultraviolet (EUV) lithography is a semiconductor fabrication technology which enables semiconductors with small features, e.g., features with dimensions of approximately 45 nanometers (nm) or less, to be produced. In EUV lithography, a laser may heat xenon gas to create a plasma, although there are other methods used to make EUV plasmas. Electrons come off the plasma and radiate light. FIG. 1 is a block diagram representation of an EUV lithography system. An EUV lithography system 100 includes a vacuum chamber 128 which contains a source 104. Source 104 provides electrons that radiate light, and includes a plasma source 108 and a collector mirror 112 that reflects electrons which come off of the plasma generated by plasma source 108 into an illuminator unit 116b of a body 116 of EUV lithography system 100. Illuminator unit 116b is a condenser that effectively collects light and directs or otherwise focuses the light onto a reticle 120. That is, illuminator unit 116b conditions light from plasma source 108 to improve uniformity. The light reflects off of reticle 120, through projection optics 116a of body 116, and onto a surface of a wafer 124.
Mirrors included in an EUV lithography system generally absorb some of light or radiation that comes into contact with the mirrors. Such mirrors (not shown) may be associated with illuminator unit 116b as well as with projection optics 116a. When light is absorbed by a mirror, the absorbed light is converted to heat. Heat generally causes distortion in the mirror, thereby degrading the optical performance of the mirror. When there is distortion in the mirror, the optical performance of the mirror is compromised.
The removal of heat from the mirrors is critical to ensure that an EUV lithography system performs with accuracy. When mirrors become heated, the mirrors may become distorted. The distortion of mirrors reduces the accuracy with which an EUV lithography system may perform.
Often, radiation is used to provide cooling to mirrors. Although radiant cooling methods may be effective in cooling mirrors while causing a relatively insignificant amount of distortion in the mirrors, when the heat load on a mirror is relatively high, radiant cooling methods are often inadequate for cooling mirrors. By way of example, radiant cooling methods are often inadequate when mirror temperatures and heat sink temperatures are not allowed to deviate greatly from the overall system temperature. In particular, radiant cooling methods generally do not provide sufficient cooling when higher power densities are involved.
Internal or direct cooling methods, e.g., liquid cooling methods, may be applied to mirrors to provide cooling in the presence of relatively high heat loads. Conventional internal cooling methods are typically associated with turbulent flow, as turbulent flow provides for relatively efficient heat transfer and cooling. However, the use of turbulent or non-laminar flow to cool a mirror generally causes the mirror to vibrate. When a mirror vibrates, the vibrations caused by the turbulent flow may effectively adversely affect a lithography process, particularly if the vibrations cause a heat exchanger, an illuminator unit, projection optics, a reticle, or a wafer to vibrate. By way of example, the accuracy of the EUV lithography process may be compromised when optics vibrate or are otherwise distorted.
An EUV lithography process preferably uses efficient, high performance heat exchangers and low complexity, low cost mirror assemblies. Mirror assemblies that are relatively complex are difficult to exchange, e.g., because coolant paths to the mirror assemblies are substantially severed in order to exchange mirror assemblies. In general, however, internally cooled mirrors designed for vacuum environments are relatively complex and expensive.
Therefore, what is needed is a relatively low cost method and an apparatus which allows heat to be efficiently removed from a mirror used in a EUV lithography system without adversely affecting the accuracy of a EUV lithography process. That is, what is desired is a method and an apparatus which efficiently removes heat from a mirror used in a EUV lithography system while substantially minimizing vibrations transmitted to optical elements which are structurally connected to the mirror.